The present invention relates generally to electronic ballasts for powering a discharge lamp. More particularly, the present invention relates to an electronic ballast for suppressing discharge lamp flicker during a transition from lamp startup to steady-state operation.
Conventionally, an electronic ballast for lighting a hot-cathode discharge lamp such as a high pressure discharge lamp (also called HID (high-intensity discharge lamp) is provided with a power converter for converting input DC power to output AC power, a resonant circuit connected between output terminals of the power converter, along with the discharge lamp, and a controller for controlling the power converter.
In an example of such an electronic ballast as known in the art, the controller executes a starting operation for raising an output voltage of the power converter relatively higher to start the discharge lamp, and then begins a steady-state operation in which the power converter is controlled to output to the lamp the AC power for maintaining the lighting of the discharge lamp.
More specifically, the starting operation makes the discharge lamp output a high starting voltage by setting an output frequency of the power converter (hereinafter referred to as an “operating frequency”) to a resonant frequency of the resonant circuit and the discharge lamp (hereinafter referred to as a “load circuit”) with the discharge lamp unlit, or to approximately 1/(n) of the resonant frequency over a predetermined starting time, where “n” is an odd number greater than 3.
Here, the resonant frequency of the load circuit changes in accordance with the beginning of the discharge of the discharge lamp, i.e., the starting thereof. Then, when an operation frequency during the starting operation is far from the resonant frequency of the load circuit after the starting of the discharge lamp, the electric power supplied to the discharge lamp by the end of the starting operation relatively decreases, thereby relatively lowering the temperature of each electrode of the discharge lamp. Therefore, the discharge becomes unstable at the time of beginning the steady-state operation, which may generate lamp flicker and an imperfect lighting.